1. Field of the Invention
The present invention relates to a method for producing high-magnetic-flux-density grain-oriented electrical steel sheet such as is used for the cores of transformers.
2. Description of the Prior Art
Grain-oriented electrical steel sheet is a mildly magnetic material used in various items of electrical equipment, such as, for example, transformers. For this, it is required that such materials exhibit good magnetic properties; specifically, excitation properties and a low watt loss.
Generally B.sub.8 is used to represent excitation properties numerically, the said B.sub.8 being the magnetic flux density at a field strength of 800A/m. Watt loss is generally indicated as W.sub.17/50, which is the watt loss per kilogram of material magnetised to 1.7 T at 50 Hz.
The grain-oriented electrical steel sheet is obtained by producing secondary recrystallization during the final-annealing process thereby to achieve a so-called Goss orientation, i.e., {110} plane &lt;001&gt; axis. To obtain good magnetic properties, it is important that the axis of easy magnetization, i.e., &lt;001&gt;, be aligned to a high degree with the rolling orientation of the sheet. The thickness of the sheet, grain diameter, specific resistance, surface film and the degree of purity of the sheet also have a major bearing on magnetic properties.
Orientation was improved considerably by a process characterized by the use of final high-reduction cold-rolling utilizing MnS or AlN as inhibitors, and by the accompanying marked improvement in core loss.
With the sharp increases in energy prices of recent years, transformer manufacturers have re-doubled their efforts to find materials from which they could construct low-core-loss transformers. Amorphous alloys and 6.5% silicon steel are among materials which are being developed for low-loss cores. However, these materials still have problems which need to be solved if the materials are to be used for transformers on an industrial basis.
Recent years have also seen the development of magnetic domain control techniques utilizing lasers, and these techniques have resulted in dramatic improvements in core-loss properties. Furthermore, the higher the magnetic flux density of the product is, the greater the effectiveness of the magnetic domain control techniques, which has increased the necessity of developing products with very high magnetic flux densities.
Japanese published unexamined patent application No. 62(1987)-222024 proposed a method for raising the magnetic flux density during the production of grain-oriented electrical steel sheet containing Al. This method comprised increasing the N.sub.2 partial pressure of the annealing atmosphere at an intermediate stage between the start and finish of secondary recrystallization. However, stable production of heavy coils weighing from five to twenty tons is difficult.
Another method for raising the magnetic flux density during the production of grain-oriented electrical steel sheet containing Al was proposed in Japanese published examined patent application No. 56(1981)-33450 which comprised lowering the rate at which the temperature is increased during final finish-annealing. However, owing to instabilities in secondary recrystallization with this method, there are still problems to be solved before it can be applied industrially.